The routine child immunization schedule is widely claimed to be the most successful public health intervention of the past century. Despite this success, parental vaccine hesitancy is widespread. Vaccine hesitancy may lead parents to obtain a non-medical or personal beliefs exemptions from school-entry immunization mandates. In California, the focus of this study, personal beliefs exemptions (PBEs) quadrupled from 1996 to 2010. Incidence of vaccine- preventable childhood diseases (VPCDs) is also on the rise. While higher rates of exemptions have been shown to be associated with increased outbreak risk of outbreak of VPCDs, the strength of this association and its implications depend critically on the true vaccination status of children with exemptions. However, there is scant evidence on the vaccination status of exempted children, making exemption prevalence a noisy signal for outbreak risk. Current approaches to assessing outbreak risk using exemption prevalence likely misestimate that risk. At the same time, little is known about school-level practices that may promote exemptions independent of parental preferences. The goal of this study is to improve estimates of the disease outbreak risk associated with exemptions. The central hypothesis is that the vaccination status of exempted children varies by school-level exemption rates and by school-level practices related to tracking exemptions. Two specific aims will test this hypothesis: (1) Estimate the vaccination status of California kindergarteners with personal beliefs exemptions. (2) Identify school practices associated with higher vs. lower exemption rates. The study will use child-level immunization data and school-level immunization tracking practices routinely collected by the California Department of Public Health to conduct secondary analyses of the vaccination status of exempted children and the association of school procedures with PBE rates. Bounded estimates and Latin Hypercube Sampling techniques will be used to address missing data. The expected outcomes of this study include the first population-level estimates of the vaccination status of exempted kindergarteners in a large state. These estimates can be used to assess the actual disease outbreak risk associated with PBEs, and to reduce the risk of VPCDs associated with the clustering of vaccine refusal within school communities. The study will also identify school-level practices that may be good candidates for interventions to reduce exemptions. The long-term goal of this program of research is to preserve herd immunity against VPCDs.